In the mounting of electrical fixtures, especially large coachlight types of fixtures, in new or retrofit applications, there is often a problem with obtaining an aesthetically acceptable mounting because of a mismatch between the electrical box installed, or to be installed, and the general configuration of the base of the electrical fixture. This is particularly so in the case where an electrical fixture is being fitted over an existing electrical box and clapboard or some similar siding material with an irregular surface. In such a situation, the normally flat vertical surface of the fixture base does not fit suitably against the irregularities of the siding, leaving gaps which are aesthetically unacceptable. Additionally, the location of the bracket mounting holes in such fixtures may vary by as much as plus or minus one inch, thus making vertical centering of the fixture on any mounting plate intervening between the base of the fixture and the outer surface of the structure even more difficult.